1. Field of the Invention
The present invention relates to an optical functional device and to an optical integrated device that have a periodic structure with refractive indices varying in recurring periodic patterns, and that can such as multiplex and demultiplex optical signals.
2. Description of the Prior Art
The structure of an optical functional device having a periodic structure composed of media of different refractive indices is shown in FIG. 1. The optical functional device 1 is so constructed that, in a medium 3, cylindrical media 2 are arranged in a predetermined two-dimensional periodic pattern. For example, by forming holes in a silicon substrate (hereinafter, referred to as a Si substrate) in predetermined recurring periodic patterns, the media 2, 3 can form the optical functional device 1 composed of the media having different refractive indices, namely air and silicon.
The thus obtained optical functional device 1 is generally called as a photonic crystal, and by properly selecting the refractive indices of the media 2, 3, the shapes of the media 2, 3 such as cylindrical or prismatic, the types of the lattice such as a trigonal lattice or a tetragonal lattice, or the periodicity in arranging the media 2, 3, it is possible to obtain different optical properties corresponding to light having particular wavelengths or polarization directions.
FIG. 2 shows a conventional optical integrated device 10 provided with the optical functional device 1. On a substrate 8, the optical functional device 1 is arranged, and waveguides 4, 5, 6 are formed for guiding light entering the optical functional device 1 and light exiting from the optical functional device 1. When, via an input terminal 4a of the waveguide 4, light consisting of components having wavelengths of λ1 and λ2 is inputted, the light enters the optical functional device 1 through the waveguide 4. From the optical functional device 1, the light components are outputted in the different directions in accordance with their wavelengths, and via terminals 5a, 6a of the waveguides 5, 6, the light components having wavelengths of λ1 and λ2 are outputted, respectively.
Therefore, it is possible to use the optical integrated device 10 as a demultiplexer for demultiplexing optical signals of multiplex light having different wavelengths, and as a multiplexer for multiplexing light having different wavelengths by reversing the direction in which light travels. By making transmittance for light having a predetermined wavelength higher and transmittance for light having other wavelengths lower, it is also possible to use the optical integrated device 10 as a filter for transmitting light having a specific wavelength. Furthermore, when the light entering the optical functional device 1 contains TE-polarized and TM-polarized light that has different polarization directions, it is also possible to output the light components in the different directions or intercept them according to their polarization directions.
A conventional laser beam printer and copying machine can perform printing in the cross direction of paper, with using a rotatable polygon mirror, by scanning the light reflected on the surrounding surfaces of the polygon mirror. However, the polygon mirror is required to perform with high accuracy, and this makes it costly. In addition, there is a limitation to accelerate the rotation speed thereof, and this causes a problem in accelerating the printing speed.
Therefore, C. S. Tsai, IEEE Trans. Circuits and Systems, vol. CAS-26, no. 12, 1979 discloses an optical scanning apparatus in which an optical functional device using a waveguide having a diffraction grating is provided for scanning light by varying the exiting directions in accordance with the wavelengths of incident light.
However, in the conventional optical functional device 1 and the optical integrated device 10 described earlier, the refractive indices and the shapes of media 2, 3, the types of lattice, or the periodic pattern thereof are selected according to light having a predetermined wavelength or polarization direction. Therefore, if the light to be used have wavelengths or polarization directions different from those of other light to be used, it requires separate designing and manufacturing processes, and this makes the optical functional device 1 and the optical integrated device 10 costly. Furthermore, the conventional optical functional device 1 and the optical integrated device 10 can be used as a multiplexer, a demultiplexer, and the like; however, they do not possess a function as a switching device that transmits specific light at a predetermined period and that intercepts the light at a predetermined period.
In addition, in respect to the optical scanning apparatus having the conventional diffraction grating mentioned above, the diffraction grating exhibits a small range of refractive index distribution, and this causes a problem of a small scanning angle. Furthermore, because of the difficulty in securing enough light amount of the light dispersed by the diffraction grating, it is difficult to put this optical scanning apparatus into practical use.